What’s that? A very close view of the network of an aspen (Populus tremuloides) leaf. The entire image is 1.3 mm x 1.0 mm. The little circular structures between the veins are individual cell walls!

This network looks very different than the one I showed you last time. It branches and reconnects with itself in intriguing ways, which we’re just starting to be able to describe. Several decades ago Leo Hickey and others came up with a classification scheme for leaf networks, which now finds form in the Manual of Leaf Architecture. But be warned – you’ll need to be a dedicated botanist to get through the terminology (semicraspedodromous secondaries, anyone?)

Let’s focus on something much simpler for now – the total density of veins, or how much of the leaf’s mass is made of veins (and not other tissue). More veins, more carbon, and a more expensive leaf – but also, more veins, more water supply, and a more functional leaf that can better take carbon from the atmosphere. How does a plant balance this cost against this benefit? Well, it depends on the environment. You might imagine that in deserts it’s better to have a high-cost leaf that doesn’t lose much water, whereas in a temperate forest (e.g. where an aspen would live) it might be better to have a low-cost leaf that can take up a lot of carbon quickly. These ideas let us begin to understand why leaves (and their networks) look so different across the world!